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Universal Serial Bus

Unlike other serial interface technologies such as RS232 that is free to use, USB is not. If you want to design a USB product then you have to pay a 1500$ fee each year for getting ID numbers to use with your USB device for operational.

USB got four wires.




The USB hardware select between full speed usb2.0 and lowspeed usb1.0 by examine if any of the data wires D+ or D- got a pullup resistor connected.

1.5Kohm connected to D+ from 3.3V = fullspeed USB

1.5Kohm connected to D- from 3.3V = lowspeed USB

The USB-host is a little dictator that gives each device a slice of each juicy 1ms timeslot that is distributed to the USB-devices.

USB-devices can be powered from the USB cable, if the USB-device feel suicidial when connected then it can ask if the USB-host wants to send over some go-up-in-smoke frying current. (500 mA max)

HackerPort - IO-Pins via USB for hardware hacking